The present invention relates to a magnetic tape cassette used in a video tape recorder (VTR) deck and particularly to a mechanism for locking a tape reel arranged rotatably in a cassette body.
In the conventional magnetic tape cassette used for a household VTR deck, a supply tape reel and a take-up tape reel both of which are wound with a magnetic tape are arranged rotatably in a cassette body, a part of the magnetic tape is extended over the front side of the cassette body, and a reel-locking mechanism is arranged at the rear side between both tape reels.
In aforesaid reel-locking mechanism, when the magnetic tape cassette is inserted into a VTR deck, reel-locking levers disengage from both tape reels to allow the tape reels to rotate and when the cassette is out of the VTR deck, the reel-locking levers engage with both tape reels to prevent them from rotating, thus avoiding the damage of the magnetic tape caused by its slackening. A sample of the art of this kind is shown in FIG. 3.
In Japanese Patent Publication Open to Public Inspection No. 197986/1985 representing aforesaid art, when engagement claw 21 of reel-locking lever 20 engages With serration groove 23 of tape reel 22 and thereby locks the tape reel 22, sliding surface 21a that is a part of the tip of engagement claw 21 positioned in serration groove 23 inner from the engagement point S is located on the curcular arc .gamma. whose radius is a segment of a line connecting the engagement point S and a center of supporting shaft 24 that is a center for swiveling of reel-locking lever 20, or the sliding surface 21a is located inside the circular arc .gamma. as shown in FIG. 3, thus, it is possible to prevent so-called over-locking state wherein the engagement claw 21 does not disengage from the serration groove 23, and to assure the smooth disengagement.
However, there has been a problem in aforesaid mechanism that the engagement claw of the reel-locking lever easily and accidentally comes off the serration groove when the tape reel is touched and moved through the opening on the bottom of cassette body or the magnetic tape cassette itself is given a shock.